A standard addition method is known as a method for finding out the proportion (composition ratio) of a specific component contained in a mixture made by mixing diverse components. In the following, a description is given taking a solution as an example. In the standard addition method, in order to know an unknown composition ratio (concentration) of a specific component (hereinafter referred to as a component targeted for quantification) in a solution sample, a component identical to the component targeted for quantification is used as a solute and is added to the sample stepwise with the concentration of the component being changed.
Signal intensities obtained by applying spectrometry to the sample in the case where the concentration of the solute added is zero and in the respective cases where the concentration of the solute added increases stepwise are plotted with respect to the solute-addition concentration, thereby enabling a so-called calibration curve to be obtained. The calibration curve can be approximated substantially linearly, and thus can be extrapolated to a range within which the solute-addition concentration has negative values to enable determination of an addition concentration of a negative value which corresponds to a signal intensity value of zero. The absolute value of the determined value is equal to the unknown concentration before the addition of the component targeted for quantification is started. In this way, the unknown concentration of the component targeted for quantification can be found out (see, for example, PTL 1).
PTL 1, noted above, describes a method in which the concentration of carbon having specific bonds that bring a preferable effect to the properties of an electrode catalyst, which is used for a fuel cell or the like, in the catalyst is determined by Raman spectroscopy. Specifically, a method is presented in which the target sample to be quantified is split into four or more containers and a component identical to the component targeted for quantification, called furnace black, is added to each of the containers in differing concentrations to prepare a relation curve between the Raman spectroscopic intensity and the concentration.
While the Raman spectroscopic analysis is used as spectroscopic analysis in PTL 1, the type of spectroscopic analysis available in the standard addition method is not limited to the Raman spectroscopic analysis. An appropriate type of spectroscopic analysis is selectable on a case-by-case basis taking into account conditions such as the properties of the object to be measured, the required measurement accuracy, and the availability of measurement facilities.
However, existing standard addition methods (including the method described in PTL 1) are designed to add a component identical to the component targeted for quantification to the sample stepwise and to perform spectroscopic measurement and analysis each time the component is added to the sample, and thus have drawbacks of being labor-intensive and taking a comparatively long time. The existing standard addition methods also have a drawback of being typically difficult to apply when the sample is a solid.